1. Field of the Invention
The present invention relates to a method and system for reducing ElectroMagnetic Interference (EMI) emissions from system clock signals or other signals.
2. Description of the Related Art
ElectroMagnetic Interference (EMI) is electromagnetic radiation which is emitted by electrical circuits carrying rapidly changing signals, as a by-product of their normal operation, and which causes unwanted signals (interference or noise) to be induced in other circuits. This interrupts, obstructs, or otherwise degrades or limits the effective performance of those other circuits. Typically, EMI is induced unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like.
EMI may affect AM or FM radio reception and television reception. As a result, many countries regulate the levels of EMI emissions that are allowed to emanate from electronic equipment and require testing of such equipment to verify compliance with the regulations. For example, a typical setup for performing EMI testing is shown in FIG. 1. Equipment under test 102 may be connected to test control equipment 104, which controls the operation of equipment 102 for purposes of the test, and to test equipment 106, which may be need to be connected to equipment 102 in order to allow equipment 102 to operate normally for the test. EMI emissions 108 emanate from equipment under test 102 and are received by antenna 110. EMI tester 112 is connected to antenna 110 and provides measurements of the levels of EMI emissions 108 as received by antenna 110.
In order to comply with EMI regulations, many techniques for reducing EMI have been developed. Examples of conventional EMI reduction techniques include the use of bypass or “decoupling” capacitors on each active device (connected across the power supply, as close to the device as possible), filtered connectors (feed-through capacitors), shielded cables, risetime control of high speed signals using series resistors, spread spectrum oscillators, and VCC filtering. Shielding is usually a last resort after other techniques have failed because of the added expense of RF gaskets and the like. However, all of these techniques increase the cost of the electronic equipment involved, as additional electronic and mechanical components must be added to otherwise fully functional equipment. A need arises for a reduced-cost technique by which EMI emissions may be reduced.